In this type of heat exchanger, the inlet and outlet tube branches are generally joined to the heat exchanger body at one face of the latter. The space available outside and adjacent to this face is limited, so that the tube branches need to be bent immediately adjacent to that face of the body, so as then to extend away from the body in a direction substantially parallel to the face to which they are joined.
The minimum radius of curvature that can be given to a tube of circular transverse cross section without either rupture or dangerous deformation occurring, is an increasing function of its diameter.
The minimum size of the tube branches in the direction at right angles to the associated face of the body is thus determined by the diameter of the tube branches, and therefore by their fluid passage cross sectional area.